The army reportedly used automated phone calls over the weekend to warn Gaza residents to say away from buildings harboring "terrorist infrastructure." From the AP:

A senior military official confirmed that Israel has a policy of striking at buildings containing Hamas operational centers or those from which military activities are launched. The official said each strike required prior approval from military lawyers and is carried out only after the local population is warned.

Israeli media reports the 12-story apartment tower housed a fourth-floor apartment where Hamas ran an operations center. According to the AP, the military declined to comment on why it collapsed the entire building instead of targeting a specific apartment.

Prime Minister Benjamin Netanyahu warned Gaza residents to stay away from Hamas militants on Sunday at a cabinet meeting, saying, "I call on the people of Gaza to immediately evacuate any structure that Hamas is using to commit acts of terror. Every one of these structures is a target for us."